Magnetic field sensors in the form of so-called “proximity detectors” that can detect the presence of a ferromagnetic object proximate to the magnetic field sensor are known. Proximity detectors typically include a permanent magnet to generate a magnetic field and also include a magnetic field sensing element, for example, a Hall effect element, to detect changes in the strength of the magnetic field associated with the permanent magnet as a ferromagnetic object moves through the magnetic field.
The output signal of a magnetic field sensing element is dependent upon the strength of a magnetic field that the magnetic field sensing element experiences. Therefore, the magnetic field sensing element can detect a distance between the proximity detector and a ferromagnetic object within the magnetic field generated by a permanent magnet. The range over which the ferromagnetic object can be detected is limited by the flux density, i.e., the strength of the magnetic field.
Where it is desired to determine the speed or rotational position of a rotating object, such as a disk mounted on a shaft, the object can be provided with ferromagnetic surface features, such as teeth, that project toward the proximity detector. The proximity of a tooth to the proximity detector tends to increase the strength of the magnetic field proximate to a proximity detector. Accordingly, by monitoring the output of the proximity detector, the rotational speed of the disk can be determined by correlating the peaks in the output of the proximity detector with the known number of teeth on the circumference of the disk. Similarly, when the teeth are irregularly spaced in a predetermined pattern, the rotational position of the object can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
One type of proximity detector uses a Hall effect element. The Hall effect element is typically mounted so that is has a maximum response axis directed toward the object to be sensed. The associated magnet is mounted in a position to achieve a magnetic field aligned generally along the maximum response axis of the Hall effect element. The object to be sensed can be a high magnetic permeability component that can have projecting surface features, which increase the strength of the magnet's magnetic field as the distance between the surface of the object and the permanent magnet is reduced. While one form of object can be a gear, another form of object can be a segmented ring magnet. Yet another form of object does not rotate at all, but merely moves closer to or further away from the proximity detector. The object to be sensed moves relative to the stationary Hall effect element within the proximity detector, and in doing so, causes the magnetic flux through the Hall effect element to vary in a manner corresponding to the position of the object. With the change in magnet flux, there occurs the corresponding change in magnet field strength, which increases (or alternatively, decreases) the output signal from the Hall effect element.
It will be understood that, within an integrated proximity detector, a position or spacing of the magnet relative to the magnetic field sensing element, e.g., the Hall effect element, greatly influences the sensitivity of the proximity detector. Therefore, it is desirable that the spacing be close and that spacing be consistent device to device.
With the increasing sophistication of products, proximity detectors have become common in automobile control systems. Examples of automotive proximity detectors include proximity detectors that detect ignition timing from a position of an engine crankshaft and/or camshaft, and the proximity detectors that detect a position or rotation and a speed of rotation of an automobile wheel for anti-lock braking systems and four wheel steering systems.
A common shortcoming of proximity detectors is their dependence upon the distance, known as the air gap, between the object to be sensed and the magnetic field sensing element within the proximity detector. More specifically, as the air gap increases, the output of a Hall effect element within the proximity detector, which is directly proportional to the strength of the magnetic field, decreases, making it more difficult to accurately analyze the output of the Hall effect element.
Conventionally, the air gap is defined as a distance between the object to be sensed and the outer surface of the package containing the proximity detector. However, as used herein, the term “effective air gap” is used to describe a distance between the object to be sensed and the magnetic field sensing element, e.g., Hall effect element, within the packaged proximity detector.
Some forms of proximity detectors that package a magnet and a Hall effect element together are described in U.S. Pat. No. 5,963,028, issued Oct. 5, 1999, and U.S. Pat. No. 6,265,865, issued Jul. 24, 2001, which are incorporated herein by reference in their entirety.
It is known that a magnet is relatively expensive. The manufacture of conventional forms of proximity detectors does not allow the magnet to be reused or replaced once the molding step is completed. Thus, if a conventional proximity detector fails manufacturing testing after molding, the cost of the magnet is lost in addition to the cost of the semiconductor die and packaging materials.
It would be desirable to provide a packaging scheme for a proximity detector (or magnetic field sensor) that would provide reliable protection from the environment, that would avoid an excessive increase in the effective air gap between the associated magnetic field sensing element and the object to be sensed, that would allow the magnetic field sensing element to be as close as possible to the magnet, and for which a proximity detector that fails testing during manufacture need not result in a loss of the magnet.
Other forms of proximity detectors include a magnet apart from an integrated proximity detector. Other forms of magnetic field sensors employ no magnet at all, but instead sense an external magnetic field experienced by the magnetic field sensor. All of these forms of magnetic field sensors would also benefit from the above characteristics.